Method and apparatus for encoding residual block, and method and apparatus for decoding residual block

ABSTRACT

A decoding apparatus includes a splitter which splits the image into a plurality of maximum coding units, hierarchically splits a maximum coding unit among the plurality of maximum coding units into a plurality of coding units, and determines one or more transformation residual blocks from a coding unit among the plurality of coding units, wherein the one or more transformation residual blocks include sub residual blocks, a parser which obtains an effective coefficient flag of a sub residual block among the sub residual blocks from a bitstream, the effective coefficient flag of the sub residual block indicating whether at least one non-zero effective transformation coefficient exists in the sub residual block, and when the effective coefficient flag indicates that at least one non-zero transformation coefficient exists in the sub residual block, obtains transformation coefficients of the sub residual block based on location information of the non-zero transformation coefficient and level information of the non-zero transformation coefficient obtained from the bitstream, and an inverse-transformer which performs inverse-transformation on a transformation residual block including the sub residual block based on the transformation coefficients of the sub residual block.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/914,248 filed Oct. 28, 2010, which claims priority from Korean PatentApplication No. 10-2009-0102818, filed on Oct. 28, 2009 in the KoreanIntellectual Property Office, the entire disclosures of the priorapplications are incorporated herein by reference in their entireties.

BACKGROUND

1. Field

Apparatuses and methods consistent with exemplary embodiments relate toencoding and decoding, and more particularly, to encoding and decodingof a residual block.

2. Description of the Related Art

As hardware for reproducing and storing high resolution or high qualityvideo content is being developed and supplied, a need for a video codecfor effectively encoding or decoding the high resolution or high qualityvideo content is increasing. In a related art video codec, a video isencoded according to a limited prediction mode based on a macroblockhaving a predetermined size. Also, the related art video codec encodes aresidual block by using a transformation unit having a small size, suchas 4×4 or 8×8.

SUMMARY

Exemplary embodiments provide a method and apparatus for efficientlyencoding and decoding effective transformation coefficient informationin a transformation residual block having a large size.

According to an aspect of an exemplary embodiment, there is provided amethod of encoding a residual block, the method including: generating aprediction block of a current block; generating a residual block basedon a difference between the prediction block and the current block;generating a transformation residual block by transforming the residualblock to a frequency domain; splitting the transformation residual blockinto frequency band units; and encoding effective coefficient flagsindicating frequency band units, of the split frequency band units, inwhich nonzero effective transformation coefficients exist.

According to an aspect of another exemplary embodiment, there isprovided an apparatus for encoding a residual block, the apparatusincluding: a predictor which generates a prediction block of a currentblock; a subtractor which generates a residual block based on adifference between the prediction block and the current block; atransformer which generates a transformation residual block bytransforming the residual block to a frequency domain; an entropyencoder which splits the transformation residual block into frequencyband units, and encodes effective coefficient flags indicating frequencyband units, of the split frequency band units, in which nonzeroeffective transformation coefficients exist.

According to an aspect of another exemplary embodiment, there isprovided a method of decoding a residual block, the method including:extracting effective coefficient flags from an encoded bitstream, theeffective coefficient flags indicating frequency band units in whichnonzero effective transformation coefficients exist, from among splitfrequency band units obtained by splitting a transformation residualblock of a current block; splitting the transformation residual blockinto the split frequency band units; and determining a frequency bandunit having an effective transformation coefficient from among the splitfrequency band units obtained by splitting the transformation residualblock, by using the extracted effective coefficient flags.

According to an aspect of another exemplary embodiment, there isprovided an apparatus for decoding a residual block, the apparatusincluding: a parser which extracts effective coefficient flags from anencoded bitstream, the effective coefficient flags indicating frequencyband units in which nonzero effective transformation coefficients exist,from among split frequency band units obtained by splitting atransformation residual block of a current block; and an entropy decoderwhich splits the transformation residual block into the split frequencyband units, and determines a frequency band unit having an effectivetransformation coefficient from among the split frequency band unitsobtained by splitting the transformation residual block, by using theextracted effective coefficient flags.

According to an aspect of another exemplary embodiment, there isprovided a method of encoding a residual block, the method including:generating a transformation residual block by transforming a residualblock to a frequency domain; splitting the transformation residual blockinto frequency band units; and encoding effective coefficient flagsindicating frequency band units, of the frequency band units, in whichnonzero effective transformation coefficients exist.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and/or other aspects will become more apparent by describingin detail exemplary embodiments with reference to the attached drawingsin which:

FIG. 1 is a block diagram of an apparatus for encoding a video,according to an exemplary embodiment;

FIG. 2 is a block diagram of an apparatus for decoding a video,according to an exemplary embodiment;

FIG. 3 is a diagram for describing a concept of coding units accordingto an exemplary embodiment;

FIG. 4 is a block diagram of an image encoder based on coding unitsaccording to an exemplary embodiment;

FIG. 5 is a block diagram of an image decoder based on coding unitsaccording to an exemplary embodiment;

FIG. 6 is a diagram illustrating deeper coding units according todepths, and partitions according to an exemplary embodiment;

FIG. 7 is a diagram for describing a relationship between a coding unitand transformation units, according to an exemplary embodiment;

FIG. 8 is a diagram for describing encoding information of coding unitscorresponding to a coded depth, according to an exemplary embodiment;

FIG. 9 is a diagram of deeper coding units according to depths,according to an exemplary embodiment;

FIGS. 10 through 12 are diagrams for describing a relationship betweencoding units, prediction units, and transformation units, according toone or more exemplary embodiments;

FIG. 13 is a diagram for describing a relationship between a codingunit, a prediction unit or a partition, and a transformation unit,according to encoding mode information of exemplary Table 1 below,according to an exemplary embodiment;

FIGS. 14A through 14C are reference diagrams for describing a process ofencoding a transformation residual block in a related technical field;

FIG. 15 is a block diagram of an apparatus for encoding a residualblock, according to an exemplary embodiment;

FIGS. 16A through 16J are diagrams for describing splitting of atransformation residual block into predetermined frequency band units,according to one or more exemplary embodiments;

FIGS. 17A and 17B are reference diagrams for describing a process ofencoding an effective transformation coefficient, according to one ormore exemplary embodiments;

FIGS. 18A and 18B are reference diagrams for describing in detail aprocess of encoding a residual block, according to an exemplaryembodiment;

FIGS. 19A and 19B are reference diagrams for describing encodinginformation of a transformation residual block, which is generated by aneffective coefficient encoder, according to one or more exemplaryembodiments;

FIG. 20 is a flowchart illustrating a method of encoding a residualblock, according to an exemplary embodiment;

FIG. 21 is a block diagram of an apparatus for decoding a residualblock, according to an exemplary embodiment; and

FIG. 22 is a flowchart illustrating a method of decoding a residualblock, according to an exemplary embodiment.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, exemplary embodiments will be described more fully withreference to the accompanying drawings. It is understood thatexpressions such as “at least one of,” when preceding a list ofelements, modify the entire list of elements and do not modify theindividual elements of the list.

In the exemplary embodiments, a “coding unit” is an encoding data unitin which the image data is encoded at an encoder side and an encodeddata unit in which the encoded image data is decoded at a decoder side.Also, a “coded depth” refers to a depth where a coding unit is encoded.

FIG. 1 is a block diagram of a video encoding apparatus 100, accordingto an exemplary embodiment. Referring to FIG. 1, the video encodingapparatus 100 includes a maximum coding unit splitter 110, a coding unitdeterminer 120, and an output unit 130.

The maximum coding unit splitter 110 may split a current picture of animage based on a maximum coding unit for the current picture. If thecurrent picture is larger than the maximum coding unit, image data ofthe current picture may be split into the at least one maximum codingunit. The maximum coding unit according to an exemplary embodiment maybe a data unit having a size of 32×32, 64×64, 128×128, 256×256, etc.,wherein a shape of the data unit is a square having a width and lengthin squares of 2. The image data may be output to the coding unitdeterminer 120 according to the at least one maximum coding unit.

A coding unit according to an exemplary embodiment may be characterizedby a maximum size and a depth. The depth denotes a number of times thecoding unit is spatially split from the maximum coding unit, and as thedepth deepens, deeper encoding units according to depths may be splitfrom the maximum coding unit to a minimum coding unit. A depth of themaximum coding unit is an uppermost depth and a depth of the minimumcoding unit is a lowermost depth. Since a size of a coding unitcorresponding to each depth decreases as the depth of the maximum codingunit deepens, a coding unit corresponding to an upper depth may includea plurality of coding units corresponding to lower depths.

As described above, the image data of the current picture is split intothe maximum coding units according to a maximum size of the coding unit,and each of the maximum coding units may include deeper coding unitsthat are split according to depths. Since the maximum coding unitaccording to an exemplary embodiment is split according to depths, theimage data of a spatial domain included in the maximum coding unit maybe hierarchically classified according to depths.

A maximum depth and a maximum size of a coding unit, which limit thetotal number of times a height and a width of the maximum coding unitcan be hierarchically split, may be predetermined.

The coding unit determiner 120 encodes at least one split regionobtained by splitting a region of the maximum coding unit according todepths, and determines a depth to output encoded image data according tothe at least one split region. That is, the coding unit determiner 120determines a coded depth by encoding the image data in the deeper codingunits according to depths, based on the maximum coding unit of thecurrent picture, and selecting a depth having the least encoding error.Thus, the encoded image data of the coding unit corresponding to thedetermined coded depth is output to the output unit 130. Also, thecoding units corresponding to the coded depth may be regarded as encodedcoding units.

The determined coded depth and the encoded image data according to thedetermined coded depth are output to the output unit 130.

The image data in the maximum coding unit is encoded based on the deepercoding units corresponding to at least one depth equal to or below themaximum depth, and results of encoding the image data are compared basedon each of the deeper coding units. A depth having the least encodingerror may be selected after comparing encoding errors of the deepercoding units. At least one coded depth may be selected for each maximumcoding unit.

The size of the maximum coding unit is split as a coding unit ishierarchically split according to depths, and as the number of codingunits increases. Also, even if coding units correspond to a same depthin one maximum coding unit, it is determined whether to split each ofthe coding units corresponding to the same depth to a lower depth bymeasuring an encoding error of the image data of each coding unit,separately. Accordingly, even when image data is included in one maximumcoding unit, the image data is split to regions according to the depthsand the encoding errors may differ according to regions in the onemaximum coding unit, and thus the coded depths may differ according toregions in the image data. Therefore, one or more coded depths may bedetermined in one maximum coding unit, and the image data of the maximumcoding unit may be divided according to coding units of at least onecoded depth.

Accordingly, the coding unit determiner 120 may determine coding unitshaving a tree structure included in the maximum coding unit. The codingunits having a tree structure according to an exemplary embodimentinclude coding units corresponding to a depth determined to be the codeddepth, from among deeper coding units included in the maximum codingunit. A coding unit of a coded depth may be hierarchically determinedaccording to depths in the same region of the maximum coding unit, andmay be independently determined in different regions. Similarly, a codeddepth in a current region may be independently determined from a codeddepth in another region.

A maximum depth according to an exemplary embodiment is an index relatedto a number of splitting times from a maximum coding unit to a minimumcoding unit. A first maximum depth according to an exemplary embodimentmay denote a total number of splitting times from the maximum codingunit to the minimum coding unit. A second maximum depth according to anexemplary embodiment may denote a total number of depth levels from themaximum coding unit to the minimum coding unit. For example, when adepth of the maximum coding unit is 0, a depth of a coding unit in whichthe maximum coding unit is split once may be set to 1, and a depth of acoding unit in which the maximum coding unit is split twice may be setto 2. Here, if the minimum coding unit is a coding unit in which themaximum coding unit is split four times, 5 depth levels of depths 0, 1,2, 3 and 4 exist. Thus, the first maximum depth may be set to 4, and thesecond maximum depth may be set to 5.

Prediction encoding and transformation may be performed according to themaximum coding unit. The prediction encoding and the transformation arealso performed based on the deeper coding units according to a depthequal to or depths less than the maximum depth, based on the maximumcoding unit. Transformation may be performed according to a method oforthogonal transformation or integer transformation.

Since the number of deeper coding units increases whenever the maximumcoding unit is split according to depths, encoding such as theprediction encoding and the transformation is performed on all of thedeeper coding units generated as the depth deepens. For convenience ofdescription, the prediction encoding and the transformation willhereinafter be described based on a coding unit of a current depth, in amaximum coding unit.

The video encoding apparatus 100 may variously select at least one of asize and a shape of a data unit for encoding the image data. In order toencode the image data, operations, such as prediction encoding,transformation, and entropy encoding, may be performed, and at thistime, the same data unit may be used for all operations or differentdata units may be used for each operation.

For example, the video encoding apparatus 100 may select a coding unitfor encoding the image data and a data unit different from the codingunit so as to perform the prediction encoding on the image data in thecoding unit.

In order to perform prediction encoding in the maximum coding unit, theprediction encoding may be performed based on a coding unitcorresponding to a coded depth, i.e., based on a coding unit that is nolonger split to coding units corresponding to a lower depth.Hereinafter, the coding unit that is no longer split and becomes a basisunit for prediction encoding will be referred to as a prediction unit. Apartition obtained by splitting the prediction unit may include aprediction unit or a data unit obtained by splitting at least one of aheight and a width of the prediction unit.

For example, when a coding unit of 2N×2N (where N is a positive integer)is no longer split and becomes a prediction unit of 2N×2N, a size of apartition may be 2N×2N, 2N×N, N×2N, or N×N. Examples of a partition typeinclude symmetrical partitions that are obtained by symmetricallysplitting at least one of a height and a width of the prediction unit,partitions obtained by asymmetrically splitting the height or the widthof the prediction unit (such as 1:n or n:1), partitions that areobtained by geometrically splitting the prediction unit, and partitionshaving arbitrary shapes.

A prediction mode of the prediction unit may be at least one of an intramode, a inter mode, and a skip mode. For example, the intra mode or theinter mode may be performed on the partition of 2N×2N, 2N×N, N×2N, orN×N. In this case, the skip mode may be performed only on the partitionof 2N×2N. The encoding is independently performed on one prediction unitin a coding unit, thereby selecting a prediction mode having a leastencoding error.

The video encoding apparatus 100 may also perform the transformation onthe image data in a coding unit based on the coding unit for encodingthe image data and on a data unit that is different from the codingunit.

In order to perform the transformation in the coding unit, thetransformation may be performed based on a data unit having a sizesmaller than or equal to the coding unit. For example, the data unit forthe transformation may include a data unit for an intra mode and a dataunit for an inter mode.

A data unit used as a base of the transformation will hereinafter bereferred to as a transformation unit. A transformation depth indicatinga number of splitting times to reach the transformation unit bysplitting the height and the width of the coding unit may also be set inthe transformation unit. For example, in a current coding unit of 2N×2N,a transformation depth may be 0 when the size of a transformation unitis also 2N×2N, may be 1 when each of the height and width of the currentcoding unit is split into two equal parts, totally split into 4̂1transformation units, and the size of the transformation unit is thusN×N, and may be 2 when each of the height and width of the currentcoding unit is split into four equal parts, totally split into 4̂2transformation units, and the size of the transformation unit is thusN/2×N/2. For example, the transformation unit may be set according to ahierarchical tree structure, in which a transformation unit of an uppertransformation depth is split into four transformation units of a lowertransformation depth according to hierarchical characteristics of atransformation depth.

Similar to the coding unit, the transformation unit in the coding unitmay be recursively split into smaller sized regions, so that thetransformation unit may be determined independently in units of regions.Thus, residual data in the coding unit may be divided according to thetransformation having the tree structure according to transformationdepths.

Encoding information according to coding units corresponding to a codeddepth uses information about the coded depth and information related toprediction encoding and transformation. Accordingly, the coding unitdeterminer 120 determines a coded depth having a least encoding errorand determines a partition type in a prediction unit, a prediction modeaccording to prediction units, and a size of a transformation unit fortransformation.

Coding units according to a tree structure in a maximum coding unit anda method of determining a partition, according to exemplary embodiments,will be described in detail later with reference to FIGS. 3 through 12.

The coding unit determiner 120 may measure an encoding error of deepercoding units according to depths by using Rate-Distortion Optimizationbased on Lagrangian multipliers.

The output unit 130 outputs the image data of the maximum coding unit,which is encoded based on the at least one coded depth determined by thecoding unit determiner 120, and information about the encoding modeaccording to the coded depth, in bitstreams.

The encoded image data may be obtained by encoding residual data of animage.

The information about the encoding mode according to the coded depth mayinclude at least one of information about the coded depth, the partitiontype in the prediction unit, the prediction mode, and the size of thetransformation unit.

The information about the coded depth may be defined by using splitinformation according to depths, which indicates whether encoding isperformed on coding units of a lower depth instead of a current depth.If the current depth of the current coding unit is the coded depth,image data in the current coding unit is encoded and output. In thiscase, the split information may be defined to not split the currentcoding unit to a lower depth. Alternatively, if the current depth of thecurrent coding unit is not the coded depth, the encoding is performed onthe coding unit of the lower depth. In this case, the split informationmay be defined to split the current coding unit to obtain the codingunits of the lower depth.

If the current depth is not the coded depth, encoding is performed onthe coding unit that is split into the coding unit of the lower depth.In this case, since at least one coding unit of the lower depth existsin one coding unit of the current depth, the encoding is repeatedlyperformed on each coding unit of the lower depth, and thus the encodingmay be recursively performed for the coding units having the same depth.

Since the coding units having a tree structure are determined for onemaximum coding unit, and information about at least one encoding mode isdetermined for a coding unit of a coded depth, information about atleast one encoding mode may be determined for one maximum coding unit.Also, a coded depth of the image data of the maximum coding unit may bedifferent according to locations since the image data is hierarchicallysplit according to depths, and thus information about the coded depthand the encoding mode may be set for the image data.

Accordingly, the output unit 130 may assign encoding information about acorresponding coded depth and an encoding mode to at least one of thecoding unit, the prediction unit, and a minimum unit included in themaximum coding unit.

The minimum unit according to an exemplary embodiment is a rectangulardata unit obtained by splitting the minimum coding unit of the lowermostdepth by 4. Alternatively, the minimum unit may be a maximum rectangulardata unit that may be included in all of the coding units, predictionunits, partition units, and transformation units included in the maximumcoding unit.

For example, the encoding information output through the output unit 130may be classified into encoding information according to coding unitsand encoding information according to prediction units. The encodinginformation according to the coding units may include the informationabout the prediction mode and the size of the partitions. The encodinginformation according to the prediction units may include informationabout an estimated direction of an inter mode, a reference image indexof the inter mode, a motion vector, a chroma component of an intra mode,and an interpolation method of the intra mode. Also, information about amaximum size of the coding unit defined according to pictures, slices,or GOPs, and information about a maximum depth may be inserted into atleast one of a Sequence Parameter Set (SPS) or a header of a bitstream.

In the video encoding apparatus 100, the deeper coding unit may be acoding unit obtained by dividing at least one of a height and a width ofa coding unit of an upper depth, which is one layer above, by two. Forexample, when the size of the coding unit of the current depth is 2N×2N,the size of the coding unit of the lower depth may be N×N. Also, thecoding unit of the current depth having the size of 2N×2N may includemaximum 4 of the coding unit of the lower depth.

Accordingly, the video encoding apparatus 100 may form the coding unitshaving the tree structure by determining coding units having an optimumshape and an optimum size for each maximum coding unit, based on thesize of the maximum coding unit and the maximum depth determinedconsidering characteristics of the current picture. Also, since encodingmay be performed on each maximum coding unit by using any one of variousprediction modes and transformations, an optimum encoding mode may bedetermined considering characteristics of the coding unit of variousimage sizes.

Thus, if an image having high resolution or a large amount of data isencoded in a related art macroblock, a number of macroblocks per pictureexcessively increases. Accordingly, a number of pieces of compressedinformation generated for each macroblock increases, and thus it isdifficult to transmit the compressed information and data compressionefficiency decreases. However, by using the video encoding apparatus 100according to an exemplary embodiment, image compression efficiency maybe increased since a coding unit is adjusted while consideringcharacteristics of an image and increasing a maximum size of a codingunit while considering a size of the image.

FIG. 2 is a block diagram of a video decoding apparatus 200, accordingto an exemplary embodiment.

Referring to FIG. 2, the video decoding apparatus 200 includes areceiver 210, an image data and encoding information extractor 220, andan image data decoder 230. Definitions of various terms, such as acoding unit, a depth, a prediction unit, and a transformation unit, andinformation about various encoding modes for various operations of thevideo decoding apparatus 200 are similar to those described above withreference to FIG. 1.

The receiver 210 receives and parses a bitstream of an encoded video.The image data and encoding information extractor 220 extracts encodedimage data for each coding unit from the parsed bitstream, wherein thecoding units have a tree structure according to each maximum codingunit, and outputs the extracted image data to the image data decoder230. The image data and encoding information extractor 220 may extractinformation about a maximum size of a coding unit of a current picturefrom a header about the current picture or an SPS.

Also, the image data and encoding information extractor 220 extractsinformation about a coded depth and an encoding mode for the codingunits having a tree structure according to each maximum coding unit,from the parsed bitstream. The extracted information about the codeddepth and the encoding mode is output to the image data decoder 230.That is, the image data in a bit stream is split into the maximum codingunit so that the image data decoder 230 decodes the image data for eachmaximum coding unit.

The information about the coded depth and the encoding mode according tothe maximum coding unit may be set for information about at least onecoding unit corresponding to the coded depth, and information about anencoding mode may include information about at least one of a partitiontype of a corresponding coding unit corresponding to the coded depth, aprediction mode, and a size of a transformation unit. Also, splittinginformation according to depths may be extracted as the informationabout the coded depth.

The information about the coded depth and the encoding mode according toeach maximum coding unit extracted by the image data and encodinginformation extractor 220 is information about a coded depth and anencoding mode determined to generate a minimum encoding error when anencoder, such as a video encoding apparatus 100 according to anexemplary embodiment, repeatedly performs encoding for each deepercoding unit based on depths according to each maximum coding unit.Accordingly, the video decoding apparatus 200 may restore an image bydecoding the image data according to a coded depth and an encoding modethat generates the minimum encoding error.

Since encoding information about the coded depth and the encoding modemay be assigned to a predetermined data unit from among a correspondingcoding unit, a prediction unit, and a minimum unit, the image data andencoding information extractor 220 may extract the information about thecoded depth and the encoding mode according to the predetermined dataunits. The predetermined data units to which the same information aboutthe coded depth and the encoding mode is assigned may be the data unitsincluded in the same maximum coding unit.

The image data decoder 230 restores the current picture by decoding theimage data in each maximum coding unit based on the information aboutthe coded depth and the encoding mode according to the maximum codingunits. For example, the image data decoder 230 may decode the encodedimage data based on the extracted information about the partition type,the prediction mode, and the transformation unit for each coding unitfrom among the coding units having the tree structure included in eachmaximum coding unit. A decoding process may include a predictionincluding intra prediction and motion compensation, and an inversetransformation. Inverse transformation may be performed according to amethod of inverse orthogonal transformation or inverse integertransformation.

The image data decoder 230 may perform at least one of intra predictionand motion compensation according to a partition and a prediction modeof each coding unit, based on the information about the partition typeand the prediction mode of the prediction unit of the coding unitaccording to coded depths.

Also, the image data decoder 230 may perform inverse transformationaccording to each transformation unit in the coding unit, based on theinformation about the size of the transformation unit of the coding unitaccording to coded depths, so as to perform the inverse transformationaccording to maximum coding units.

The image data decoder 230 may determine at least one coded depth of acurrent maximum coding unit by using split information according todepths. If the split information indicates that image data is no longersplit in the current depth, the current depth is a coded depth.Accordingly, the image data decoder 230 may decode encoded data of atleast one coding unit corresponding to the each coded depth in thecurrent maximum coding unit by using at least one of the informationabout the partition type of the prediction unit, the prediction mode,and the size of the transformation unit for each coding unitcorresponding to the coded depth, and output the image data of thecurrent maximum coding unit.

For example, data units including the encoding information having thesame split information may be gathered by observing the encodinginformation set assigned for the predetermined data unit from among thecoding unit, the prediction unit, and the minimum unit, and the gathereddata units may be considered to be one data unit to be decoded by theimage data decoder 230 in the same encoding mode.

The video decoding apparatus 200 may obtain information about at leastone coding unit that generates the minimum encoding error when encodingis recursively performed for each maximum coding unit, and may use theinformation to decode the current picture. That is, the coding unitshaving the tree structure determined to be the optimum coding units ineach maximum coding unit may be decoded. Also, the maximum size of thecoding unit may be determined considering at least one of resolution andan amount of image data.

Accordingly, even if image data has high resolution and a large amountof data, the image data may be efficiently decoded and restored by usinga size of a coding unit and an encoding mode, which are adaptivelydetermined according to characteristics of the image data, andinformation about an optimum encoding mode received from an encoder.

A method of determining coding units having a tree structure, aprediction unit, and a transformation unit, according to one or moreexemplary embodiments, will now be described with reference to FIGS. 3through 13.

FIG. 3 is a diagram for describing a concept of coding units accordingto an exemplary embodiment.

A size of a coding unit may be expressed in width×height. For example,the size of the coding unit may be 64×64, 32×32, 16×16, or 8×8. A codingunit of 64×64 may be split into partitions of 64×64, 64×32, 32×64, or32×32, and a coding unit of 32×32 may be split into partitions of 32×32,32×16, 16×32, or 16×16, a coding unit of 16×16 may be split intopartitions of 16×16, 16×8, 8×16, or 8×8, and a coding unit of 8×8 may besplit into partitions of 8×8, 8×4, 4×8, or 4×4.

Referring to FIG. 3, there is exemplarily provided first video data 310with a resolution of 1920×1080, and a coding unit with a maximum size of64 and a maximum depth of 2. Furthermore, there is exemplarily providedsecond video data 320 with a resolution of 1920×1080, and a coding unitwith a maximum size of 64 and a maximum depth of 3. Also, there isexemplarily provided third video data 330 with a resolution of 352×288,and a coding unit with a maximum size of 16 and a maximum depth of 1.The maximum depth shown in FIG. 3 denotes a total number of splits froma maximum coding unit to a minimum decoding unit.

If a resolution is high or a data amount is large, a maximum size of acoding unit may be large so as to increase encoding efficiency and toaccurately reflect characteristics of an image. Accordingly, the maximumsize of the coding unit of the first and the second video data 310 and320 having the higher resolution than the third video data 330 may be64.

Since the maximum depth of the first video data 310 is 2, coding units315 of the first video data 310 may include a maximum coding unit havinga long axis size of 64, and coding units having long axis sizes of 32and 16 since depths are deepened to two layers by splitting the maximumcoding unit twice. Meanwhile, since the maximum depth of the third videodata 330 is 1, coding units 335 of the third video data 330 may includea maximum coding unit having a long axis size of 16, and coding unitshaving a long axis size of 8 since depths are deepened to one layer bysplitting the maximum coding unit once.

Since the maximum depth of the second video data 320 is 3, coding units325 of the second video data 320 may include a maximum coding unithaving a long axis size of 64, and coding units having long axis sizesof 32, 16, and 8 since the depths are deepened to 3 layers by splittingthe maximum coding unit three times. As a depth deepens, detailedinformation may be precisely expressed.

FIG. 4 is a block diagram of an image encoder 400 based on coding units,according to an exemplary embodiment.

The image encoder 400 may perform operations of a coding unit determiner120 of a video encoding apparatus 100 according to an exemplaryembodiment to encode image data. That is, referring to FIG. 4, an intrapredictor 410 performs intra prediction on coding units, from among acurrent frame 405, in an intra mode, and a motion estimator 420 and amotion compensator 425 perform inter estimation and motion compensationon coding units, from among the current frame, in an inter mode by usingthe current frame 405 and a reference frame 495.

Data output from the intra predictor 410, the motion estimator 420, andthe motion compensator 425 is output as a quantized transformationcoefficient through a transformer 430 and a quantizer 440. The quantizedtransformation coefficient is restored as data in a spatial domainthrough an inverse quantizer 460 and an inverse transformer 470, and therestored data in the spatial domain is output as the reference frame 495after being post-processed through a deblocking unit 480 and a loopfiltering unit 490. The quantized transformation coefficient may beoutput as a bitstream 455 through an entropy encoder 450.

In order for the image encoder 400 to be applied in the video encodingapparatus 100, elements of the image encoder 400, i.e., the intrapredictor 410, the motion estimator 420, the motion compensator 425, thetransformer 430, the quantizer 440, the entropy encoder 450, the inversequantizer 460, the inverse transformer 470, the deblocking unit 480, andthe loop filtering unit 490, perform operations based on each codingunit from among coding units having a tree structure while consideringthe maximum depth of each maximum coding unit.

Specifically, the intra predictor 410, the motion estimator 420, and themotion compensator 425 determine partitions and a prediction mode ofeach coding unit from among the coding units having a tree structurewhile considering a maximum size and a maximum depth of a currentmaximum coding unit, and the transformer 430 determines the size of thetransformation unit in each coding unit from among the coding unitshaving a tree structure.

FIG. 5 is a block diagram of an image decoder 500 based on coding units,according to an exemplary embodiment.

Referring to FIG. 5, a parser 510 parses encoded image data to bedecoded and information about encoding used for decoding from abitstream 505. The encoded image data is output as inverse quantizeddata through an entropy decoder 520 and an inverse quantizer 530, andthe inverse quantized data is restored to image data in a spatial domainthrough an inverse transformer 540.

An intra predictor 550 performs intra prediction on coding units in anintra mode with respect to the image data in the spatial domain, and amotion compensator 560 performs motion compensation on coding units inan inter mode by using a reference frame 585.

The image data in the spatial domain, which passed through the intrapredictor 550 and the motion compensator 560, may be output as arestored frame 595 after being post-processed through a deblocking unit570 and a loop filtering unit 580. Also, the image data that ispost-processed through the deblocking unit 570 and the loop filteringunit 580 may be output as the reference frame 585.

In order to decode the image data in an image data decoder 230 of avideo decoding apparatus 200 according to an exemplary embodiment, theimage decoder 500 may perform operations that are performed after theparser 510.

In order for the image decoder 500 to be applied in the video decodingapparatus 200, elements of the image decoder 500, i.e., the parser 510,the entropy decoder 520, the inverse quantizer 530, the inversetransformer 540, the intra predictor 550, the motion compensator 560,the deblocking unit 570, and the loop filtering unit 580, performoperations based on coding units having a tree structure for eachmaximum coding unit.

Specifically, the intra prediction 550 and the motion compensator 560perform operations based on partitions and a prediction mode for each ofthe coding units having a tree structure, and the inverse transformer540 performs operations based on a size of a transformation unit foreach coding unit.

FIG. 6 is a diagram illustrating deeper coding units according todepths, and partitions, according to an exemplary embodiment.

A video encoding apparatus 100 and a video decoding apparatus 200according to exemplary embodiments use hierarchical coding units so asto consider characteristics of an image. A maximum height, a maximumwidth, and a maximum depth of coding units may be adaptively determinedaccording to the characteristics of the image, or may be differently setby a user. Sizes of deeper coding units according to depths may bedetermined according to the predetermined maximum size of the codingunit.

Referring to FIG. 6, in a hierarchical structure 600 of coding units,according to an exemplary embodiment, the maximum height and the maximumwidth of the coding units are each 64, and the maximum depth is 4. Sincea depth deepens along a vertical axis of the hierarchical structure 600,a height and a width of a deeper coding unit are each split. Also, aprediction unit and partitions, which are bases for prediction encodingof each deeper coding unit, are shown along a horizontal axis of thehierarchical structure 600.

That is, a first coding unit 610 is a maximum coding unit in thehierarchical structure 600, wherein a depth is 0 and a size, i.e., aheight by width, is 64×64. The depth deepens along the vertical axis,and a second coding unit 620 having a size of 32×32 and a depth of 1, athird coding unit 630 having a size of 16×16 and a depth of 2, a fourthcoding unit 640 having a size of 8×8 and a depth of 3, and a fifthcoding unit 650 having a size of 4×4 and a depth of 4 exist. The fifthcoding unit 650 having the size of 4×4 and the depth of 4 is a minimumcoding unit.

The prediction unit and the partitions of a coding unit are arrangedalong the horizontal axis according to each depth. That is, if the firstcoding unit 610 having the size of 64×64 and the depth of 0 is aprediction unit, the prediction unit may be split into partitionsincluded in the first coding unit 610, i.e., a partition 610 having asize of 64×64, partitions 612 having a size of 64×32, partitions 614having a size of 32×64, or partitions 616 having a size of 32×32.

Similarly, a prediction unit of the second coding unit 620 having thesize of 32×32 and the depth of 1 may be split into partitions includedin the second coding unit 620, i.e., a partition 620 having a size of32×32, partitions 622 having a size of 32×16, partitions 624 having asize of 16×32, and partitions 626 having a size of 16×16.

Similarly, a prediction unit of the third coding unit 630 having thesize of 16×16 and the depth of 2 may be split into partitions includedin the third coding unit 630, i.e., a partition having a size of 16×16included in the third coding unit 630, partitions 632 having a size of16×8, partitions 634 having a size of 8×16, and partitions 636 having asize of 8×8.

Similarly, a prediction unit of the fourth coding unit 640 having thesize of 8×8 and the depth of 3 may be split into partitions included inthe fourth coding unit 640, i.e., a partition having a size of 8×8included in the fourth coding unit 640, partitions 642 having a size of8×4, partitions 644 having a size of 4×8, and partitions 646 having asize of 4×4.

The fifth coding unit 650 having the size of 4×4 and the depth of 4 isthe minimum coding unit and a coding unit of the lowermost depth. Aprediction unit of the fifth coding unit 650 is only assigned to apartition having a size of 4×4.

In order to determine the at least one coded depth of the coding unitsof the maximum coding unit 610, a coding unit determiner 120 of thevideo encoding apparatus 100 performs encoding for coding unitscorresponding to each depth included in the maximum coding unit 610.

A number of deeper coding units according to depths including data inthe same range and the same size increases as the depth deepens. Forexample, four coding units corresponding to a depth of 2 are used tocover data that is included in one coding unit corresponding to a depthof 1. Accordingly, in order to compare encoding results of the same dataaccording to depths, the coding unit corresponding to the depth of 1 andfour coding units corresponding to the depth of 2 are each encoded.

In order to perform encoding for a current depth from among the depths,a least encoding error may be selected for the current depth byperforming encoding for each prediction unit in the coding unitscorresponding to the current depth, along the horizontal axis of thehierarchical structure 600. Alternatively, the minimum encoding errormay be searched for by comparing the least encoding errors according todepths, by performing encoding for each depth as the depth deepens alongthe vertical axis of the hierarchical structure 600. A depth and apartition having the minimum encoding error in the first coding unit 610may be selected as the coded depth and a partition type of the firstcoding unit 610.

FIG. 7 is a diagram for describing a relationship between a coding unit710 and transformation units 720, according to an exemplary embodiment.

A video encoding or decoding apparatus 100 or 200 according to exemplaryembodiments encodes or decodes an image according to coding units havingsizes smaller than or equal to a maximum coding unit for each maximumcoding unit. Sizes of transformation units for transformation duringencoding may be selected based on data units that are not larger than acorresponding coding unit.

For example, in the video encoding or decoding apparatus 100 or 200, ifa size of the coding unit 710 is 64×64, transformation may be performedby using the transformation units 720 having a size of 32×32.

Also, data of the coding unit 710 having the size of 64×64 may beencoded by performing the transformation on each of the transformationunits having the size of 32×32, 16×16, 8×8, and 4×4, which are smallerthan 64×64, such that a transformation unit having the least codingerror may be selected.

FIG. 8 is a diagram for describing encoding information of coding unitscorresponding to a coded depth, according to an exemplary embodiment.

Referring to FIG. 8, an output unit 130 of a video encoding apparatus100 according to an exemplary embodiment may encode and transmitinformation 800 about a partition type, information 810 about aprediction mode, and information 820 about a size of a transformationunit for each coding unit corresponding to a coded depth, as informationabout an encoding mode.

The information 800 about the partition type is information about ashape of a partition obtained by splitting a prediction unit of acurrent coding unit, wherein the partition is a data unit for predictionencoding the current coding unit. For example, a current coding unitCU_(—)0 having a size of 2N×2N may be split into any one of a partition802 having a size of 2N×2N, a partition 804 having a size of 2N×N, apartition 806 having a size of N×2N, and a partition 808 having a sizeof N×N. Here, the information 800 about the partition type is set toindicate one of the partition 804 having a size of 2N×N, the partition806 having a size of N×2N, and the partition 808 having a size of N×N

The information 810 about the prediction mode indicates a predictionmode of each partition. For example, the information 810 about theprediction mode may indicate a mode of prediction encoding performed ona partition indicated by the information 800 about the partition type,i.e., an intra mode 812, an inter mode 814, or a skip mode 816.

The information 820 about the size of a transformation unit indicates atransformation unit to be based on when transformation is performed on acurrent coding unit. For example, the transformation unit may be a firstintra transformation unit 822, a second intra transformation unit 824, afirst inter transformation unit 826, or a second intra transformationunit 828.

An image data and encoding information extractor 220 of a video decodingapparatus 200 according to an exemplary embodiment may extract and usethe information 800, 810, and 820 for decoding, according to each deepercoding unit

FIG. 9 is a diagram of deeper coding units according to depths,according to an exemplary embodiment.

Split information may be used to indicate a change of a depth. The splitinformation indicates whether a coding unit of a current depth is splitinto coding units of a lower depth.

Referring to FIG. 9, a prediction unit 910 for prediction encoding acoding unit 900 having a depth of 0 and a size of 2N_(—)0×2N_(—)0 mayinclude partitions of a partition type 912 having a size of2N_(—)0×2N_(—)0, a partition type 914 having a size of 2N_(—)0×N_(—)0, apartition type 916 having a size of N_(—)0×2N_(—)0, and a partition type918 having a size of N_(—)0×N_(—)0. Although FIG. 9 only illustrates thepartition types 912 through 918 which are obtained by symmetricallysplitting the prediction unit 910, it is understood that a partitiontype is not limited thereto. For example, according to another exemplaryembodiment, the partitions of the prediction unit 910 may includeasymmetrical partitions, partitions having a predetermined shape, andpartitions having a geometrical shape.

Prediction encoding is repeatedly performed on one partition having asize of 2N_(—)0×2N_(—)0, two partitions having a size of 2N_(—)0×N_(—)0,two partitions having a size of N_(—)0×2N_(—)0, and four partitionshaving a size of N_(—)0×N_(—)0, according to each partition type. Theprediction encoding in an intra mode and an inter mode may be performedon the partitions having the sizes of 2N_(—)0×2N_(—)0, N_(—)0×2N_(—)0,2N_(—)0×N_(—)0, and N_(—)0×N_(—)0. The prediction encoding in a skipmode is performed only on the partition having the size of2N_(—)0×2N_(—)0.

Errors of encoding including the prediction encoding in the partitiontypes 912 through 918 are compared, and the least encoding error isdetermined among the partition types. If an encoding error is smallestin one of the partition types 912 through 916, the prediction unit 910may not be split into a lower depth.

For example, if the encoding error is the smallest in the partition type918, a depth is changed from 0 to 1 to split the partition type 918 inoperation 920, and encoding is repeatedly performed on coding units 930having a depth of 2 and a size of N_(—)0×N_(—)0 to search for a minimumencoding error.

A prediction unit 940 for prediction encoding the coding unit 930 havinga depth of 1 and a size of 2N_(—)1×2N_(—)1 (=N_(—)0×N_(—)0) may includepartitions of a partition type 942 having a size of 2N_(—)1×2N_(—)1, apartition type 944 having a size of 2N_(—)1×N_(—)1, a partition type 946having a size of N_(—)1×2N_(—)1, and a partition type 948 having a sizeof N_(—)1×N_(—)1.

As an example, if an encoding error is the smallest in the partitiontype 948, a depth is changed from 1 to 2 to split the partition type 948in operation 950, and encoding is repeatedly performed on coding units960, which have a depth of 2 and a size of N_(—)2×N_(—)2 to search for aminimum encoding error.

When a maximum depth is d, split operations according to each depth maybe performed up to when a depth becomes d−1, and split information maybe encoded as up to when a depth is one of 0 to d−2. For example, whenencoding is performed up to when the depth is d−1 after a coding unitcorresponding to a depth of d−2 is split in operation 970, a predictionunit 990 for prediction encoding a coding unit 980 having a depth of d−1and a size of 2N_(d−1)×2N_(d−1) may include partitions of a partitiontype 992 having a size of 2N_(d−1)×2N_(d−1), a partition type 994 havinga size of 2N_(d−1)×N_(d−1), a partition type 996 having a size ofN_(d−1)×2N_(d−1), and a partition type 998 having a size ofN_(d−1)×N_(d−1).

Prediction encoding may be repeatedly performed on one partition havinga size of 2N_(d−1)×2N_(d−1), two partitions having a size of2N_(d−1)×N_(d−1), two partitions having a size of N_(d−1)×2N_(d−1), fourpartitions having a size of N_(d−1)×N_(d−1) from among the partitiontypes 992 through 998 to search for a partition type having a minimumencoding error.

Even when the partition type 998 has the minimum encoding error, since amaximum depth is d, a coding unit CU_(d−1) having a depth of d−1 is nolonger split to a lower depth. In this case, a coded depth for thecoding units of a current maximum coding unit 900 is determined to bed−1 and a partition type of the current maximum coding unit 900 may bedetermined to be N_(d−1)×N_(d−1). Also, since the maximum depth is d anda minimum coding unit 980 having a lowermost depth of d−1 is no longersplit to a lower depth, split information for the minimum coding unit980 is not set.

A data unit 999 may be a minimum unit for the current maximum codingunit. A minimum unit according to an exemplary embodiment may be arectangular data unit obtained by splitting a minimum coding unit 980 by4. By performing the encoding repeatedly, a video encoding apparatus 100according to an exemplary embodiment may select a depth having the leastencoding error by comparing encoding errors according to depths of thecoding unit 900 to determine a coded depth, and set a correspondingpartition type and a prediction mode as an encoding mode of the codeddepth.

As such, the minimum encoding errors according to depths are compared inall of the depths of 1 through d, and a depth having the least encodingerror may be determined as a coded depth. The coded depth, the partitiontype of the prediction unit, and the prediction mode may be encoded andtransmitted as information about an encoding mode. Also, since a codingunit is split from a depth of 0 to a coded depth, split information ofthe coded depth is set to 0, and split information of depths excludingthe coded depth is set to 1.

An image data and encoding information extractor 220 of a video decodingapparatus 200 according to an exemplary embodiment may extract and usethe information about the coded depth and the prediction unit of thecoding unit 900 to decode the partition 912. The video decodingapparatus 200 may determine a depth, in which split information is 0, asa coded depth by using split information according to depths, and useinformation about an encoding mode of the corresponding depth fordecoding.

FIGS. 10 through 12 are diagrams for describing a relationship betweencoding units 1010, prediction units 1060, and transformation units 1070,according to one or more exemplary embodiments.

Referring to FIG. 10, the coding units 1010 are coding units having atree structure, corresponding to coded depths determined by a videoencoding apparatus 100 according to an exemplary embodiment, in amaximum coding unit. Referring to FIGS. 11 and 12, the prediction units1060 are partitions of prediction units of each of the coding units1010, and the transformation units 1070 are transformation units of eachof the coding units 1010.

When a depth of a maximum coding unit is 0 in the coding units 1010,depths of coding units 1012 and 1054 are 1, depths of coding units 1014,1016, 1018, 1028, 1050, and 1052 are 2, depths of coding units 1020,1022, 1024, 1026, 1030, 1032, and 1048 are 3, and depths of coding units1040, 1042, 1044, and 1046 are 4.

In the prediction units 1060, some encoding units 1014, 1016, 1022,1032, 1048, 1050, 1052, and 1054 are obtained by splitting coding unitsof the encoding units 1010. In particular, partition types in the codingunits 1014, 1022, 1050, and 1054 have a size of 2N×N, partition types inthe coding units 1016, 1048, and 1052 have a size of N×2N, and apartition type of the coding unit 1032 has a size of N×N. Predictionunits and partitions of the coding units 1010 are smaller than or equalto each coding unit.

Transformation or inverse transformation is performed on image data ofthe coding unit 1052 in the transformation units 1070 in a data unitthat is smaller than the coding unit 1052. Also, the coding units 1014,1016, 1022, 1032, 1048, 1050, and 1052 of the transformation units 1070are different from those of the prediction units 1060 in terms of sizesand shapes. That is, the video encoding and decoding apparatuses 100 and200 according to exemplary embodiments may perform intra prediction,motion estimation, motion compensation, transformation, and inversetransformation individually on a data unit in the same coding unit.

Accordingly, encoding is recursively performed on each of coding unitshaving a hierarchical structure in each region of a maximum coding unitto determine an optimum coding unit, and thus coding units having arecursive tree structure may be obtained. Encoding information mayinclude split information about a coding unit, information about apartition type, information about a prediction mode, and informationabout a size of a transformation unit. Exemplary Table 1 shows theencoding information that may be set by the video encoding and decodingapparatuses 100 and 200.

TABLE 1 Split Information 0 Split (Encoding on Coding Unit having Sizeof 2N × 2N and Current Depth of d) Information 1 Prediction PartitionType Size of Transformation Unit Repeatedly Mode Encode IntraSymmetrical Asymmetrical Split Split Coding Units Inter PartitionPartition Information 0 of Information 1 of having Skip Type TypeTransformation Transformation Lower Depth (Only Unit Unit of d + 1 2N ×2N) 2N × 2N 2N × nU 2N × 2N N × N 2N × N 2N × nD (Symmetrical N × 2N nL× 2N Type) N × N nR × 2N N/2 × N/2 (Asymmetrical Type)

An output unit 130 of the video encoding apparatus 100 may output theencoding information about the coding units having a tree structure, andan image data and encoding information extractor 220 of the videodecoding apparatus 200 may extract the encoding information about thecoding units having a tree structure from a received bitstream.

Split information indicates whether a current coding unit is split intocoding units of a lower depth. If split information of a current depth dis 0, a depth in which a current coding unit is no longer split into alower depth, is a coded depth. Information about a partition type,prediction mode, and a size of a transformation unit may be defined forthe coded depth. If the current coding unit is further split accordingto the split information, encoding is independently performed on splitcoding units of a lower depth.

A prediction mode may be one of an intra mode, an inter mode, and a skipmode. The intra mode and the inter mode may be defined in all partitiontypes, and the skip mode may be defined in only a partition type havinga size of 2N×2N.

The information about the partition type may indicate symmetricalpartition types having sizes of 2N×2N, 2N×N, N×2N, and N×N, which areobtained by symmetrically splitting a height or a width of a predictionunit, and asymmetrical partition types having sizes of 2N×nU, 2N×nD,nL×2N, and nR×2N, which are obtained by asymmetrically splitting theheight or the width of the prediction unit. The asymmetrical partitiontypes having the sizes of 2N×nU and 2N×nD may be respectively obtainedby splitting the height of the prediction unit in ratios of 1:3 and 3:1,and the asymmetrical partition types having the sizes of nL×2N and nR×2Nmay be respectively obtained by splitting the width of the predictionunit in ratios of 1:3 and 3:1

The size of the transformation unit may be set to be two types in theintra mode and two types in the inter mode. For example, if splitinformation of the transformation unit is 0, the size of thetransformation unit may be 2N×2N, which is the size of the currentcoding unit. If split information of the transformation unit is 1, thetransformation units may be obtained by splitting the current codingunit. Also, if a partition type of the current coding unit having thesize of 2N×2N is a symmetrical partition type, a size of atransformation unit may be N×N, and if the partition type of the currentcoding unit is an asymmetrical partition type, the size of thetransformation unit may be N/2×N/2.

The encoding information about coding units having a tree structure mayinclude at least one of a coding unit corresponding to a coded depth, acoding unit corresponding to a prediction unit, and a coding unitcorresponding to a minimum unit. The coding unit corresponding to thecoded depth may include at least one of a prediction unit and a minimumunit including the same encoding information.

Accordingly, it is determined whether adjacent data units are includedin the same coding unit corresponding to the coded depth by comparingencoding information of the adjacent data units. Also, a correspondingcoding unit corresponding to a coded depth is determined by usingencoding information of a data unit, and thus a distribution of codeddepths in a maximum coding unit may be determined.

Accordingly, if a current coding unit is predicted based on encodinginformation of adjacent data units, encoding information of data unitsin deeper coding units adjacent to the current coding unit may bedirectly referred to and used.

However, it is understood that another exemplary embodiment is notlimited thereto. For example, according to another exemplary embodiment,if a current coding unit is predicted based on encoding information ofadjacent data units, data units adjacent to the current coding unit aresearched using encoding information of the data units, and the searchedadjacent coding units may be referred for predicting the current codingunit.

FIG. 13 is a diagram for describing a relationship between a codingunit, a prediction unit or a partition, and a transformation unit,according to encoding mode information of exemplary Table 1, accordingto an exemplary embodiment.

Referring to FIG. 13, a maximum coding unit 1300 includes coding units1302, 1304, 1306, 1312, 1314, 1316, and 1318 of coded depths. Here,since the coding unit 1318 is a coding unit of a coded depth, splitinformation may be set to 0. Information about a partition type of thecoding unit 1318 having a size of 2N×2N may be set to be one of apartition type 1322 having a size of 2N×2N, a partition type 1324 havinga size of 2N×N, a partition type 1326 having a size of N×2N, a partitiontype 1328 having a size of N×N, a partition type 1332 having a size of2N×nU, a partition type 1334 having a size of 2N×nD, a partition type1336 having a size of nL×2N, and a partition type 1338 having a size ofnR×2N.

When the partition type is set to be symmetrical, i.e., the partitiontype 1322, 1324, 1326, or 1328, a transformation unit 1342 having a sizeof 2N×2N is set if split information (TU size flag) of a transformationunit is 0, and a transformation unit 1344 having a size of N×N is set ifa TU size flag is 1.

When the partition type is set to be asymmetrical, i.e., the partitiontype 1332, 1334, 1336, or 1338, a transformation unit 1352 having a sizeof 2N×2N is set if a TU size flag is 0, and a transformation unit 1354having a size of N/2×N/2 is set if a TU size flag is 1.

Referring to FIG. 13, the TU size flag is a flag having a value of 0 or1, although it is understood that the TU size flag is not limited to 1bit, and a transformation unit may be hierarchically split having a treestructure while the TU size flag increases from 0.

In this case, the size of a transformation unit that has been actuallyused may be expressed by using a TU size flag of a transformation unit,according to an exemplary embodiment, together with a maximum size andminimum size of the transformation unit. According to an exemplaryembodiment, a video encoding apparatus 100 is capable of encodingmaximum transformation unit size information, minimum transformationunit size information, and a maximum TU size flag. The result ofencoding the maximum transformation unit size information, the minimumtransformation unit size information, and the maximum TU size flag maybe inserted into an SPS. According to an exemplary embodiment, a videodecoding apparatus 200 may decode video by using the maximumtransformation unit size information, the minimum transformation unitsize information, and the maximum TU size flag.

For example, if the size of a current coding unit is 64×64 and a maximumtransformation unit size is 32×32, the size of a transformation unit maybe 32×32 when a TU size flag is 0, may be 16×16 when the TU size flag is1, and may be 8×8 when the TU size flag is 2.

As another example, if the size of the current coding unit is 32×32 anda minimum transformation unit size is 32×32, the size of thetransformation unit may be 32×32 when the TU size flag is 0. Here, theTU size flag cannot be set to a value other than 0, since the size ofthe transformation unit cannot be less than 32×32.

As another example, if the size of the current coding unit is 64×64 anda maximum TU size flag is 1, the TU size flag may be 0 or 1. Here, theTU size flag cannot be set to a value other than 0 or 1.

Thus, if it is defined that the maximum TU size flag isMaxTransformSizeIndex, a minimum transformation unit size isMinTransformSize, and a transformation unit size is RootTuSize when theTU size flag is 0, a current minimum transformation unit sizeCurrMinTuSize that can be determined in a current coding unit, may bedefined by Equation (1):

CurrMinTuSize=max(MinTransformSize,RootTuSize/(2̂MaxTransformSizeIndex))  (1).

Compared to the current minimum transformation unit size CurrMinTuSizethat can be determined in the current coding unit, a transformation unitsize RootTuSize when the TU size flag is 0 may denote a maximumtransformation unit size that can be selected in the system. In Equation(1), RootTuSize/(2̂MaxTransformSizeIndex) denotes a transformation unitsize when the transformation unit size RootTuSize, when the TU size flagis 0, is split a number of times corresponding to the maximum TU sizeflag. Furthermore, MinTransformSize denotes a minimum transformationsize. Thus, a smaller value from amongRootTuSize/(2̂MaxTransformSizeIndex) and MinTransformSize may be thecurrent minimum transformation unit size CurrMinTuSize that can bedetermined in the current coding unit.

According to an exemplary embodiment, the maximum transformation unitsize RootTuSize may vary according to the type of a prediction mode.

For example, if a current prediction mode is an inter mode, thenRootTuSize may be determined by using Equation (2) below. In Equation(2), MaxTransformSize denotes a maximum transformation unit size, andPUSize denotes a current prediction unit size.

RootTuSize=min(MaxTransformSize,PUSize)  (2).

That is, if the current prediction mode is the inter mode, thetransformation unit size RootTuSize when the TU size flag is 0, may be asmaller value from among the maximum transformation unit size and thecurrent prediction unit size.

If a prediction mode of a current partition unit is an intra mode,RootTuSize may be determined by using Equation (3) below. In Equation(3), PartitionSize denotes the size of the current partition unit.

RootTuSize=min(MaxTransformSize,PartitionSize)  (3).

That is, if the current prediction mode is the intra mode, thetransformation unit size RootTuSize when the TU size flag is 0 may be asmaller value from among the maximum transformation unit size and thesize of the current partition unit.

However, the current maximum transformation unit size RootTuSize thatvaries according to the type of a prediction mode in a partition unit ismerely exemplary, and another exemplary embodiment is not limitedthereto.

Hereinafter, encoding and decoding of residual block performed by theentropy encoder 450 of the video encoding apparatus 400 illustrated inFIG. 4 and the entropy decoder 520 of the video decoding apparatus 500illustrated in FIG. 5 will be described in detail. In the followingdescription, an encoding unit denotes a current encoded block in anencoding process of an image, and a decoding unit denotes a currentdecoded block in a decoding process of an image. The encoding unit andthe decoding unit are different in that the encoding unit is used in theencoding process and the decoding unit is used in the decoding. For thesake of consistency, except for a particular case, the encoding unit andthe decoding unit are referred to as a coding unit in both the encodingand decoding processes. Also, one of ordinary skill in the art wouldunderstand by the present disclosure that an intra prediction method andapparatus according to an exemplary embodiment may also be applied toperform intra prediction in a general video codec.

FIGS. 14A through 14C are reference diagrams for describing a process ofencoding a transformation residual block in a related technical field.

Referring to FIG. 14A, when a transformation residual block 1410 isgenerated by transforming a residual block, a significance map, whichindicates a location of a nonzero effective transformation coefficientin the transformation residual block 1410 while scanning transformationcoefficients in the transformation residual block 1410 according to azigzag scanning order. After scanning the transformation coefficients inthe transformation residual block 1410, level information of aneffective transformation coefficient are encoded. For example, a processof encoding a transformation residual block 1420 having a size of 4×4,as illustrated in FIG. 14B, will now be described. In FIG. 14B, it isassumed that transformation coefficients at locations indicated by X arenonzero effective transformation coefficients. Here, a significance mapindicates an effective transformation coefficient as 1 and a 0transformation coefficient as 0 from among transformation coefficientsin a residual block 1430, as shown in FIG. 14C. The significance map isscanned according to a predetermined scanning order, while contextadaptive binary arithmetic coding is performed thereon. For example,when the significance map of FIG. 14C is encoded according to a rasterscanning order, and scanning is performed from left to right and top tobottom, context adaptive binary arithmetic coding is performed on thesignificance map corresponding to an binary string of “111111110101000.”Level information of an effective coefficient, i.e., a sign and anabsolute value of the effective coefficient, is encoded after thesignificance map is encoded.

Such a process in the related technical field may be utilized forencoding a transformation residual block having a small size, such as4×4 or 8×8, but may not be suitable for encoding a transformationresidual block having a large size, such as 16×16, 32×32, or 64×64. Inparticular, if all transformation coefficients in a transformationresidual block are scanned and encoded according to the process of FIGS.14A through 14C with respect to a transformation residual block having alarge size, a length of a binary string corresponding to a significancemap may increase and encoding efficiency may deteriorate.

Accordingly, a method and apparatus for encoding a residual blockaccording to exemplary embodiments are capable of efficiently encoding atransformation residual block by splitting the transformation residualblock into predetermined frequency band units and encoding an effectivecoefficient flag according to the frequency band units, which indicateswhether a nonzero effective transformation coefficient exists for eachfrequency band unit, while encoding effective transformation coefficientinformation, i.e., a significance map and level information of aneffective coefficient, in a frequency band in which an effectivecoefficient flag according to frequency band units has a value of 1.

FIG. 15 is a block diagram of an apparatus 1500 for encoding a residualblock, according to an exemplary embodiment. While not restrictedthereto, the apparatus 1500 may correspond to the entropy encoder 450 ofFIG. 4, or may be included in the entropy encoder 450.

Referring to FIG. 15, the apparatus 1500 includes a frequency bandsplitter 1510, an effective coefficient flag generator 1520, and aneffective coefficient encoder 1530.

The frequency band splitter 1510 splits a transformation residual blockinto predetermined frequency band units. Referring back to FIG. 14A, inthe exemplary transformation residual block 1410, an upper lefttransformation coefficient has a low frequency component, and a lowerright transformation coefficient has a high frequency component. Most ofthe effective transformation coefficients of the transformation residualblock 1410 may exist in low frequency bands, and the transformationcoefficients having high frequency components may mostly have a value of0. In this case, a nonzero effective transformation coefficient fromamong the transformation coefficients of the high frequency component issparse. Specifically, distribution of effective transformationcoefficients of high frequency components may be sparser when atransformation residual block is generated by performing transformationwith a transformation unit having a size of 16×16, 32×32, 64×64, orabove, which is larger than a related art transformation unit having asize of 4×4 or 8×8, as in the image encoder 400. Accordingly, thefrequency band splitter 1510 may split the transformation residual blockinto the frequency band units while considering distributioncharacteristics according to the frequency bands of the transformationcoefficients in the transformation residual block.

FIGS. 16A through 16J are diagrams for describing splitting of atransformation residual block into predetermined frequency band units,according to one or more exemplary embodiments.

Referring to FIG. 16A, the frequency band splitter 1510 generatesfrequency band units 1611 through 1614 by splitting a transformationresidual block 1610 at predetermined frequency intervals from a lowfrequency band to a horizontal frequency H1 and a vertical frequency V1.In FIG. 16A, horizontal sides and vertical sides of the frequency bandunits 1611 through 1614 have the same length, although it is understoodthat the lengths of the horizontal and vertical sides may differ fromeach other. If a length of a remaining frequency band from thehorizontal frequency H1 to a maximum horizontal frequency is less than afrequency interval corresponding to a length of the horizontal side ofeach of the frequency band units 1611 through 1614, or if a length of aremaining frequency band from the vertical frequency V1 to a maximumvertical frequency is less than a frequency interval corresponding to alength of the vertical side of each of the frequency band units 1611through 1614, the frequency band splitter 1510 no longer splits thetransformation residual block 1610, and generates a frequency band unit1615 corresponding to a high frequency component. Effectivetransformation coefficients may be intensively distributed in thefrequency band units 1611 through 1614 corresponding to low frequencycomponents, and distribution of effective transformation coefficients ofhigh frequency components may be sparse. Accordingly, even when theentire remaining high frequency components, aside from the frequencyband units 1611 through 1614 generated by splitting the transformationresidual block 1610 at predetermined frequency intervals, are generatedin one frequency band unit 1615, an overhead while encodingtransformation coefficients in the frequency band unit 1615 may notremarkably increase.

In another exemplary embodiment, as shown in FIG. 16B, the frequencyband splitter 1510 may generate frequency band units 1621 through 1624by splitting a transformation residual block 1620 from a low frequencyband to a horizontal frequency H2 and a vertical frequency V2, andgenerate frequency band units 1625 through 1627 by splitting remaininghigh frequency components of the transformation residual block 1620based on the horizontal frequency H2 and the vertical frequency V2,similarly to the description with reference to FIG. 16A.

Moreover, according to another exemplary embodiment, as shown in FIG.16C, the frequency band splitter 1510 may generate frequency band units1631 through 1634 by splitting a transformation residual block 1630 froma low frequency band to a horizontal frequency H3 and a verticalfrequency V3, and generate frequency band units 1635 and 1636 of highfrequency components by splitting remaining high frequency components ofthe transformation residual block 1630 into two based on the verticalfrequency V3, similarly to the description with reference to FIG. 16A.

Referring to FIG. 16D, according to another exemplary embodiment, thefrequency band splitter 1510 may generate frequency band units 1641through 1644 by splitting a transformation residual block 1640 from alow frequency band to a horizontal frequency H4 and a vertical frequencyV4, and generate frequency band units 1645 and 1646 of high frequencycomponents by splitting remaining high frequency components of thetransformation residual block 1630 into two based on the horizontalfrequency H4, similarly to the description with reference to FIG. 16A.

As described above, distribution of effective transformationcoefficients is concentrated in a low frequency band, and is sparsetoward a high frequency band. Accordingly, as shown in FIG. 16E, thefrequency band splitter 1510 splits a transformation residual block 1650in such a way that a unit size split in the low frequency band issmaller than a unit size split in the high frequency band, byconsidering a distribution characteristic of the effectivetransformation coefficients. In other words, the frequency band splitter1510 splits the transformation residual block 1650 minutely in the lowfrequency band and relatively large in the high frequency band so thatthe effective transformation coefficients that are concentrated in thelow frequency band are precisely encoded. For example, as shown in FIG.16E, the frequency band splitter 1510 may generate frequency band splitunits 1651 through 1657 by splitting the transformation residual block1650 based on a horizontal frequency H5, vertical frequency V5, ahorizontal frequency H6 having a larger value than a multiple of thehorizontal frequency H5, and a vertical frequency V6 having a largervalue than a multiple of the vertical frequency V5. Thus, when A1651through A1657 respectively denote sizes of the frequency band splitunits 1651 through 1657, the transformation residual block 1650 is splitin such a way that A1651 has a minimum size and A1657 has a maximumsize.

Referring to FIG. 16F, according to another exemplary embodiment, thefrequency band splitter 1510 may split a transformation residual block1660 into frequency band units 1661 having the same size.

Moreover, referring to FIG. 16G, according to another exemplaryembodiment, the frequency band splitter 1510 may quadrisect atransformation residual block 1670, and again quadrisect a smallest lowfrequency band unit 1671 from among quadrisected frequency band units togenerate frequency band units. The frequency band splitter 1510 mayagain quadrisect a smallest low frequency band unit 1672 from amongfrequency band units obtained by quadrisecting the smallest lowfrequency band unit 1671. Such a splitting process may be repeated untilsizes of quadrisected frequency band units are equal to or below apredetermined size.

According to another exemplary embodiment, referring to FIG. 16H, thefrequency band splitter 1510 may generate a frequency band unit 1681 ofa low frequency component from a low frequency to a horizontal frequencyH7 and a vertical frequency V7, and generate frequency band units 1682and 1683 by diagonally splitting remaining high frequency components ofa transformation residual block 1680.

Referring to FIGS. 16I and 16J, according to one or more other exemplaryembodiments, the frequency band splitter 1510 may split transformationresidual blocks 1690 and 1695 by connecting a horizontal frequency and avertical frequency, which have predetermined values. In FIG. 16I, thetransformation residual block 1690 is split by connecting the horizontalfrequency and the vertical frequency at uniform frequency intervals. InFIG. 16J, the transformation residual block 1695 is split so thatfrequency intervals increase toward a high frequency, i.e., byconnecting a1 and b1, a2 and b2, a3 and b3, and a4 and b4, whereina1<a2<a3<a4 and b1<b2<b3<b4.

According to another exemplary embodiment, instead of using apredetermined split form as shown in FIGS. 16A through 16J, thefrequency band splitter 1510 may determine image characteristics of atransformation residual block by using distribution characteristics ofeffective transformation coefficients of the transformation residualblock or a number of the effective transformation coefficients in eachfrequency band, and determine a size of a frequency unit to split thetransformation residual block according to each frequency band by usingthe determined image characteristics. For example, when effectivetransformation coefficients in a transformation residual block existonly in a frequency band smaller than a horizontal frequency H8 and avertical frequency V8 and do not exist in a frequency band larger thanthe horizontal frequency H8 and the vertical frequency V8, the frequencyband splitter 1510 may set the entire transformation residual block froma low frequency band to the horizontal frequency H8 and the verticalfrequency V8 as one frequency band unit. Alternatively, the frequencyband splitter 1510 split the transformation residual block intofrequency band units having the same size, and set a remaining frequencyband larger than the horizontal frequency H8 and the vertical frequencyV8 as one frequency band unit.

It is understand that the splitting of a transformation residual blockinto predetermined frequency band units is not limited to the exemplaryembodiments described above with reference to FIGS. 16A through 16J, andthat a transformation residual block may be split into various forms inone or more other exemplary embodiments.

Meanwhile, split forms of a transformation residual block by thefrequency band splitter 1510 may be identically set in an encoder and adecoder. However, it is understood that another exemplary embodiment isnot limited thereto. For example, according to another exemplaryembodiment, a predetermined split index may be determined for each ofvarious split form, such as shown in FIGS. 16A through 16J, and theencoder may insert the split index about split information used whileencoding a transformation residual block into an encoded bitstream. Forexample, when integer values from split index (div_index) 0 to 9respectively denote split forms of FIGS. 16A through 16J, and a splitform used to encode a current transformation residual block isdiv_index=5 corresponding to the form shown in FIG. 16F, such splitinformation may be added to encoding information of the currenttransformation residual block.

Referring back to FIG. 15, after the frequency band splitter 1510 splitsthe transformation residual block into the frequency band units, theeffective coefficient flag generator 1520 generates an effectivecoefficient flag indicating whether an effective transformationcoefficient exists in each frequency band unit. Here, the effectivecoefficient flag generator 1520 may not generate a separate effectivecoefficient flag for a smallest low frequency band unit. For example,when the transformation residual block 1610 of FIG. 16A is split, theeffective coefficient flag generator 1520 may generate effectivecoefficient flags indicating whether effective transformationcoefficients exist for the frequency band units 1612 through 1615, otherthan the frequency band unit 1611 of a smallest low frequency band unit.When Coeff_exist_(—)1612, Coeff_exist_(—)1613, Coeff_exist_(—)1614, andCoeff_exist_(—)1615 respectively denote the effective coefficient flagsof the frequency band units 1612 through 1615, and effectivecoefficients exist only in the frequency band units 1612 and 1613 fromamong the frequency band units 1612 through 1615, the effectivecoefficient flag generator 1520 generates the effective coefficientflags of each frequency band unit, for example, generatesCoeff_exist_(—)1612=1, Coeff_exist_(—)1613=1, and Coeff_exist_(—)1614=0,Coeff_exist_(—)1615=0. As described above, since an effectivetransformation coefficient may exist in the frequency band unit 1611 ofthe smallest low frequency band unit, an effective coefficient flagindicating existence of the effective transformation coefficient may notbe separately generated for the frequency band unit 1611. Moreover,instead of separately generating the effective coefficient flag for thefrequency band unit 1611, a related art coded_block_flag fieldindicating whether an effective transformation coefficient exists in aresidual block may be used to indicate the existence of the effectivetransformation coefficient in the frequency band unit 1611. Such aprocess of generating the effective coefficient flag is not limited tothe slit form of FIG. 16A, and may be applied to other split forms inone or more other exemplary embodiments, such as those of FIGS. 16Bthrough 16J.

Meanwhile, transformation process or inverse-transformation process maybe performed individually in each frequency band unit by use ofdifferent transformation or inverse-transformation method. Further,transformation process or inverse-transformation may be performed onlyin the frequency band unit having an effective coefficient flag 1, andmay be skipped in the frequency band unit having an effectivecoefficient flag 0.

Referring back to FIG. 15, the effective coefficient encoder 1530encodes a significance map and level information of the effectivetransformation coefficient. The significance map indicates locations ofthe effective transformation coefficients existing in the frequency bandunit, in which a value of the effective coefficient flag generated bythe effective coefficient flag generator 1520 is 1, i.e., the frequencyband unit having the effective transformation coefficient.

FIGS. 17A and 17B are reference diagrams for describing a process ofencoding an effective transformation coefficient, according to one ormore exemplary embodiments. FIGS. 17A and 17B illustrate split formscorresponding to the split form of FIG. 16E, wherein frequency bandunits are generated by quadrisecting a transformation residual block,and again quadrisecting a low frequency band. It is understood that theprocess described with reference to FIGS. 17A and 17B may also beapplied to the frequency band units having other split forms, such asany one of the split forms of FIGS. 16A through 16J.

The effective coefficient encoder 1530 may encode an effectivetransformation coefficient by scanning an entire transformation residualblock, or encode an effective transformation coefficient in a frequencyband unit by performing scanning independently for each frequency bandunit. In detail, referring to FIG. 17A, the effective coefficientencoder 1530 may encode a significance map indicating locations ofeffective transformation coefficients existing in a transformationresidual block 1710, and size and sign information of each effectivetransformation coefficient, while scanning the entire transformationresidual block 1710 according to a predetermined scanning order, forexample, a raster scanning order as shown in FIG. 17A. Here, scanningmay be skipped in a frequency band unit in which an effectivecoefficient flag has a value of 0, i.e., a frequency band unit that doesnot have an effective transformation coefficient.

According to another exemplary embodiment, referring to FIG. 17B, theeffective coefficient encoder 1530 may encode significance map and levelinformation of an effective transformation coefficient for eachfrequency band unit according to a split form of a transformationresidual block 1720 split by the frequency band splitter 1510.

FIGS. 18A and 18B are reference diagrams for describing in detail aprocess of encoding a residual block, according to an exemplaryembodiment. In FIGS. 18A and 18B, a transformation coefficient indicatedwith x is an effective transformation coefficient, and a transformationcoefficient without any indication has a value of 0.

Referring to FIG. 18A, the frequency band splitter 1510 splits atransformation residual block 1810 according to a split form, such asone of the split forms shown in FIGS. 16A through 16J. FIG. 18A shows asplit form corresponding to the split form of FIG. 16E, though it isunderstood that the process with reference to FIG. 18A may also beapplied to other split forms. The effective coefficient flag generator1520 respectively sets effective coefficient flags of frequency bandunits 1811 through 1813 including effective transformation coefficientsas 1, and respectively sets effective coefficient flags of frequencyband units 1814 through 1817 that do not include an effectivetransformation coefficient as 0. The effective coefficient encoder 1530encodes a significance map indicating locations of the effectivetransformation coefficients while scanning the entire transformationresidual block 1810. As described above, the significance map indicateswhether a transformation coefficient according to each scan index is aneffective transformation coefficient or 0. After encoding thesignificance map, the effective coefficient encoder 1530 encodes levelinformation of each effective transformation coefficient, The levelinformation of the effective transformation coefficient includes signand absolute value information of the effective transformationcoefficient. For example, the significance map of the frequency bandunits 1811 through 1813 including the effective transformationcoefficients may have a binary string value, such as“1000100010101110100100100010001,” when scanning is performed accordingto a raster scanning order as shown in FIG. 18A.

Also, when information about the effective transformation coefficient isencoded while scanning the entire transformation residual block 1810 asshown in FIG. 18A, an end-of-block (EOB) flag indicating whether aneffective transformation coefficient is the last effectivetransformation coefficient may be set for the entire transformationresidual block 1810 or each frequency band unit. When an EOB flag is setfor the entire transformation residual block 1810, only an EOB flag of atransformation coefficient 1802 of the last effective transformationcoefficient according to the scanning order from among transformationcoefficients of FIG. 18A may have a value of 1. For example, asdescribed above, if the significance map according to FIG. 18A has avalue of “1000100010101110100100100010001,” an EOB flag corresponding tosuch a significance map has a value of “000000000001” since only thelast effective transformation coefficient from among 12 effectivetransformation coefficients included in“1000100010101110100100100010001” has a value of 1. In other words, atotal of 12 bits are used to express the EOB flag corresponding to thesignificance map of FIG. 18A.

Alternatively, in order to reduce a number of bits used to express anEOB flag, the effective coefficient encoder 1530 may define a flag(Tlast) indicating whether a last effective transformation coefficientexists according to each frequency band unit, set Tlast as 1 if the lasteffective transformation coefficient according to each frequency bandunit exists and as 0 if the last effective transformation coefficientdoes not exist, and sets an EOB flag for only a frequency band unitwhere Tlast is 1, thereby reducing a number of bits used to identifylocations of effective transformation coefficients in the entiretransformation residual block and the last effective transformationcoefficient. In detail, referring to FIG. 18A, the effective coefficientencoder 1530 may check the existence of a last effective transformationcoefficient for each of the frequency band units 1811 through 1813including the effective transformation coefficients, and set Tlast as 1in the frequency band unit 1812 including the last effectivetransformation coefficient, and set Tlast as 0 in the remainingfrequency band units 1811 and 1813. If each bit of Tlast indicates theexistence of the last effective transformation coefficient in each ofthe frequency band units 1811 through 1813 according to an order ofscanning the transformation coefficients, a most significant bit (MSB)of Tlast may indicate whether the effective transformation coefficientexists in a lowest frequency band unit, and a least significant bit(LSB) of Tlast may indicate whether the last effective transformationcoefficient exists in the frequency band unit 1812. That is, a bit valueof “001” is set since Tlast has a value of 0 for the frequency band unit1811, 0 for the frequency band unit 1813, and 1 for the frequency bandunit 1812. Here, since an effective transformation coefficient in atransformation residual block may end at the frequency band unit 1811that is the lowest, a Tlast value may not be separately assigned for thefrequency band unit 1811. That is, Tlast may be set only for thefrequency bands 1812 and 1813 excluding the frequency band 1811 fromamong the frequency band units 1811 through 1813 that are scannedaccording to a scanning order. Here, two bit values of “01” are set asTlast. “0” that is the MSB of “01” indicates that the last effectivetransformation coefficient of the transformation residual block does notexist in the frequency band unit 1813, and “1” that is the LSB of “01”indicates that the last effective transformation coefficient of thetransformation residual block exists in the frequency band unit 1812.Tlast may have a value of “00” if the last effective transformationcoefficient of the transformation residual block exists in the frequencyband 1811 of the lowest frequency band unit. Thus, when all bits ofTlast are 0, it may be determined that the last effective transformationcoefficient of the transformation residual block exists in the frequencyband unit 1811.

In the present exemplary embodiment, the effective coefficient encoder1530 sets an EOB flag only for the frequency band unit in which Tlast is1, i.e., the frequency band unit including the last effectivetransformation coefficient of the transformation residual block.Referring to FIG. 18A, the effective coefficient encoder 1530 sets anEOB flag only for each effective transformation coefficient existing inthe frequency band unit 1812 in which Tlast is 1. Since a total foureffective transformation coefficients exist in the frequency band unit1812, the EOB flag has four bits of “0001.” According to anotherexemplary embodiment, a total of six to seven bits are used to identifythe location of the effective transformation coefficients in thetransformation residual block, and the last effective transformationcoefficient, since two to three bits are set for Tlast and four bits areset for the EOB flag. Here, five to six bits are saved compared to thepreviously described exemplary embodiment in which a total of 12 bitsare used to set the EOB flag, such as “000000000001.”

According to another exemplary embodiment, when an EOB flag is set foreach frequency band unit, EOB flags of a transformation coefficient 1801in the frequency band unit 1811, a transformation coefficient 1802 inthe frequency band unit 1812, and a transformation coefficient 1803 inthe frequency band unit 1813 are set to 1. EOB flags are not set for thefrequency band units 1814 through 1817 that do not include the effectivetransformation coefficients. As such when an EOB flag is set for eachfrequency band unit including an effective transformation coefficient,an effective transformation coefficient in a predetermined frequencyband unit is scanned, and then an effective transformation coefficientin a following frequency band unit may be scanned. For example, atransformation coefficient in the frequency band unit 1812 may bescanned after the transformation coefficient 1803 of the frequency bandunit 1813 is scanned. Referring to FIG. 18B, effective transformationcoefficient information is encoded independently for each frequency bandunit. The effective coefficient encoder 1530 encodes a significance mapindicating locations of effective transformation coefficients, and levelinformation of each effective transformation coefficient whileindependently scanning each frequency band unit of a transformationresidual block 1820. For example, a significance map of a frequency bandunit 1821 has a binary string value such as “1000100010011” when scannedaccording to a raster scanning order as shown in FIG. 18B. Also, theeffective coefficient encoder 1530 sets an EOB flag of an effectivetransformation coefficient 1831 corresponding to a last effectivetransformation coefficient from among effective transformationcoefficients of the frequency band unit 1821 as 1. Similarly, theeffective coefficient encoder 1530 generates a binary string value, suchas “101010001,” as a significance map of a frequency band unit 1822.Also, the effective coefficient encoder 1530 sets an EOB of an effectivetransformation coefficient 1832 from among effective transformationcoefficients in the frequency band unit 1822 as 1. Similarly, theeffective coefficient encoder 1530 generates a binary string value, suchas “11001,” as a significance map of a frequency band unit 1823, andsets an EOB flag of an effective transformation coefficient 1833 as 1.

Meanwhile, the effective coefficient encoder 1530 may separately encodean End_Of_WholeBlock flag indicating a last effective transformationcoefficient of the transformation residual block 1820, aside from theEOB flag indicating that the effective transformation coefficients 1831through 1833 are the last effective transformation coefficients in acorresponding frequency band unit. Referring to FIG. 18B, if thefrequency band units 1821 through 1827 are independently scanned in thestated order, the effective transformation coefficient 1833 is the lasteffective transformation coefficient of the frequency band unit 1823and, at the same time, the last effective transformation coefficient ofthe transformation residual block 1820. Accordingly, an EOB flag and anEnd_Of_WholeBlock flag of the effective transformation coefficient 1833both have a value of 1. In the effective transformation coefficients1831 and 1832, which are the last effective transformation coefficientsof the frequency band units 1821 and 1822, EOB flags have a value of 1,but End_Of_WholeBlock flags have a value of 0.

As such, when an EOB flag and an End_Of_WholeBlock flag are set for alast effective transformation coefficient according to each frequencyband, existence of an effective transformation coefficient in acorresponding frequency band unit may be first determined by using anabove-described effective coefficient flag during decoding so as to skipscanning of a frequency band unit, in which an effective coefficientflag is 0. Furthermore, when a transformation coefficient, in which anEOB flag is 1, is scanned while scanning transformation coefficients ina frequency band unit, in which an effective coefficient flag is 1,i.e., a frequency band unit having an effective transformationcoefficient, a following frequency band unit may be scanned. When aneffective transformation coefficient, in which an EOB flag is 1 and anEnd_Of_WholeBlock flag is 1, is scanned, effective transformationcoefficients of an entire transformation residual block are scanned, andthus scanning of the transformation residual block is ended.

FIGS. 19A and 19B are reference diagrams for describing encodinginformation of a transformation residual block, which is generated bythe effective coefficient encoder 1530, according to one or moreexemplary embodiments.

Referring to FIG. 19A, the effective coefficient encoder 1530 maysequentially encode significance maps and pieces of effectivecoefficient flag information generated according to frequency bands.When a first frequency band is a smallest frequency band of atransformation residual block, only a significance map 1911 of the firstfrequency band may be encoded and a flag of the first frequency band,which indicates whether an effective transformation coefficient existsin the first frequency band, may not be separately encoded, as shown inFIG. 19A. According to another exemplary embodiment, referring to FIG.19B, effective coefficient flags 1921 of each frequency band may befirst encoded, and then significance maps 1925 of each frequency bandmay be encoded.

FIG. 20 is a flowchart illustrating a method of encoding a residualblock, according to an exemplary embodiment.

Referring to FIG. 20, the intra predictor 410 or the motion compensator425 of FIG. 4 generates a prediction block via inter prediction or intraprediction by using a current block in operation 2010.

In operation 2020, a subtractor generates a residual block that is adifference between the prediction block and the current block.

In operation 2030, the transformer 430 transforms the residual blockinto a frequency domain to generate a transformation residual block. Forexample, the residual block may be transformed to the frequency domainvia discrete cosine transform (DCT).

In operation 2040, the frequency band splitter 1510 splits thetransformation residual block into predetermined frequency band units.As described above, the frequency band splitter 1510 may split thetransformation residual block into one of various split forms, forexample as shown in FIGS. 16A through 16J. In detail, the frequency bandsplitter 1510 may split the transformation residual block such that aunit size split in a low frequency band is smaller than a unit sizesplit in a high frequency band, split the transformation residual blockby quadrisecting the transformation residual block and repeatedlyquadrisecting a smallest low frequency band in the quadrisectedtransformation residual block, split the transformation residual blockinto frequency band units having the same size, split the transformationresidual block by connecting a horizontal frequency and a verticalfrequency having the same value, or determine a split size according tofrequency bands of the transformation residual block by using imagecharacteristics of the transformation residual block determined by usingtransformation coefficients of the transformation residual block, andsplit the transformation residual block according to the determinedsplit size according to frequency bands.

In operation 2050, the effective coefficient flag generator 1520generates an effective coefficient flag according to frequency bandunits, wherein the effective coefficient flag indicates whether anonzero effective transformation coefficient exists in each frequencyband unit. The effective coefficient flag may not be separatelygenerated for a smallest frequency band unit from among the frequencyband units of the transformation residual block. Also, the effectivecoefficient encoder 1530 encodes a significance map indicating locationsof the effective transformation coefficients and level information ofthe effective transformation coefficients with respect to the frequencyband units, in which the effective coefficient flags are not 0, i.e.,the frequency band units including the effective transformationcoefficients, while scanning the transformation residual block accordingto a predetermined scanning order or independently scanning eachfrequency band unit, as described above with reference to FIGS. 17A,17B, 18A, and 18B.

According to a method and an apparatus for encoding a residual blockaccording to one or more exemplary embodiments as described above,information about an effective transformation coefficient may beefficiently encoded according to distribution characteristics of theeffective transformation coefficient in a transformation residual blockhaving a size that is greater than or equal to 16×16, by splitting thetransformation residual block into frequency band units. Thus, atransformation residual block having a large size is split intofrequency band units, and an effective coefficient flag indicating anexistence of the effective transformation coefficient is generatedaccording to frequency band units. Accordingly, a scanning process of afrequency band, in which an effective transformation coefficient doesnot exist in the transformation residual block, may be skipped and anumber of bits generated to encode the effective transformationcoefficient may be reduced.

FIG. 21 is a block diagram of an apparatus 2100 for decoding a residualblock, according to an exemplary embodiment. While not restrictedthereto, the apparatus 2100 may correspond to the entropy decoder 520 ofFIG. 5 or be included in the entropy decoder 520.

Referring to FIG. 21, the apparatus 2100 includes a frequency bandsplitter 2110, an effective frequency band determiner 2120, and aneffective coefficient decoder 2130.

The frequency band splitter 2110 splits a transformation residual blockinto predetermined frequency band units. In detail, as described withreference to FIGS. 16A through 16H, the frequency band splitter 2110 maysplit the transformation residual block in such a way that a unit sizesplit in a low frequency band is smaller than a unit size split in ahigh frequency band, split the transformation residual block byquadrisecting the transformation residual block and repeatedlyquadrisecting a smallest low frequency band in the quadrisectedtransformation residual block, split the transformation residual blockinto frequency band units having the same size, split the transformationresidual block by connecting a horizontal frequency and a verticalfrequency having the same value, or determine a split size according tofrequency bands of the transformation residual block by using imagecharacteristics of the transformation residual block determined by usingtransformation coefficients of the transformation residual block, andsplit the transformation residual block according to the determinedsplit size according to frequency bands. A split form of thetransformation residual block may be predetermined by an encoder and adecoder, though it is understood that another exemplary embodiment isnot limited thereto. For example, according to another exemplaryembodiment, when a predetermined split index is set for each split formand information about a split index used to split a currenttransformation residual block is added to a bitstream during encoding,the frequency band splitter 2110 may determine which split form was usedto split the current transformation residual block based on theinformation about the split index included in the bitstream.

The effective frequency band determiner 2120 extracts an effectivecoefficient flag from a bitstream, wherein the effective coefficientflag indicates whether an effective transformation coefficient existsaccording to the frequency band units obtained by splitting thetransformation residual block. The effective frequency band determiner2120 may determine a frequency band unit including an effectivetransformation coefficient from among the frequency band units by usingthe effective coefficient flag. For example, when the transformationresidual block 1820 of FIG. 18B is used, the effective coefficient flagsof the frequency band units 1821 through 1823 have a value of 1, and theeffective coefficient flags of the frequency band units 1824 through1827 have a value of 0. Thus, the effective frequency band determiner2120 may determine the frequency band units including the effectivetransformation coefficients from the extracted effective coefficientflags according to the frequency bands.

The effective coefficient decoder 2130 decodes the effectivetransformation coefficients in the frequency band units that aredetermined to include the effective transformation coefficients by theeffective frequency band determiner 2120. In detail, the effectivecoefficient decoder 2130 extracts a significance map indicatinglocations of the effective transformation coefficients and levelinformation of the effective transformation coefficients, from thebitstream. Also, as described above with reference to FIGS. 17A and 17B,the effective coefficient decoder 2130 determines the locations of theeffective transformation coefficients in the transformation residualblock by using the significance map, and restores values of theeffective transformation coefficients by using the level informationwhile scanning the entire transformation residual block or scanning eachfrequency band unit according to a predetermined scanning order that isindependent for each frequency band unit.

FIG. 22 is a flowchart illustrating a method of decoding a residualblock, according to an exemplary embodiment.

Referring to FIG. 22, in operation 2210, the effective frequency banddeterminer 2120 extracts an effective coefficient flag from an encodedbitstream, wherein the effective coefficient flag indicates whether aneffective transformation coefficient exists according to frequency bandunits obtained by splitting a transformation residual block of a currentblock.

In operation 2220, the frequency band splitter 2110 splits thetransformation residual block into the frequency band units. Asdescribed above with reference to FIGS. 16A through 16J, the frequencyband splitter 2110 may split the transformation residual block in such away that a unit size split in a low frequency band is smaller than aunit size split in a high frequency band, split the transformationresidual block by quadrisecting the transformation residual block andrepeatedly quadrisecting a smallest low frequency band in thequadrisected transformation residual block, split the transformationresidual block into frequency band units having the same size, split thetransformation residual block by connecting a horizontal frequency and avertical frequency having the same value, or determine a split sizeaccording to frequency bands of the transformation residual block byusing image characteristics of the transformation residual blockdetermined by using transformation coefficients of the transformationresidual block, and split the transformation residual block according tothe determined split size according to frequency bands. Such a splitform may be predetermined with an encoder, or may be determined by usinginformation about a split index separately added to the encodedbitstream. Moreover, it is understood that operations 2210 and 2220 maybe switched in order or performed simultaneously or substantiallysimultaneously.

In operation 2230, the frequency band splitter 2110 determines afrequency band unit including an effective transformation coefficientfrom among the frequency band units, by using the extracted effectivecoefficient flag. The effective coefficient decoder 2130 restores theeffective transformation coefficient by using a significance map aboutthe frequency band unit determined to include the effectivetransformation coefficient, and level information of the effectivetransformation coefficient.

According to one or more exemplary embodiments, an effective coefficientflag indicating existence of an effective transformation coefficient isgenerated according to frequency band units, so that a scanning processof a frequency band skips a transformation residual block in which aneffective transformation coefficient does not exist, and a number ofbits generated to encode the effective transformation coefficient isreduced.

While not restricted thereto, an exemplary embodiment can also beembodied as computer readable code on a computer readable recordingmedium. The computer readable recording medium is any data storagedevice that can store data which can be thereafter read by a computersystem. Examples of the computer readable recording medium includeread-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetictapes, floppy disks, and optical data storage devices. The computerreadable recording medium can also be distributed over network coupledcomputer systems so that the computer readable code is stored andexecuted in a distributed fashion.

While exemplary embodiments have been particularly shown and described,it will be understood by one of ordinary skill in the art that variouschanges in form and details may be made therein without departing fromthe spirit and scope of the inventive concept as defined by thefollowing claims. The exemplary embodiments should be considered in adescriptive sense only and not for purposes of limitation. Therefore,the scope of the inventive concept is defined not by the detaileddescription of the exemplary embodiments, but by the following claims,and all differences within the scope will be construed as being includedin the present inventive concept.

What is claimed is:
 1. An apparatus for decoding an image, the apparatuscomprising: a splitter which splits the image into a plurality ofmaximum coding units, hierarchically splits a maximum coding unit amongthe plurality of maximum coding units into a plurality of coding units,and determines one or more transformation residual blocks from a codingunit among the plurality of coding units, wherein the one or moretransformation residual blocks include sub residual blocks; a parserwhich obtains an effective coefficient flag of a sub residual blockamong the sub residual blocks from a bitstream, the effectivecoefficient flag of the sub residual block indicating whether at leastone non-zero effective transformation coefficient exists in the subresidual block, and when the effective coefficient flag indicates thatat least one non-zero transformation coefficient exists in the subresidual block, obtains transformation coefficients of the sub residualblock based on location information of the non-zero transformationcoefficient and level information of the non-zero transformationcoefficient obtained from the bitstream; and an inverse-transformerwhich performs inverse-transformation on a transformation residual blockincluding the sub residual block based on the transformationcoefficients of the sub residual block, wherein the transformationcoefficients of the sub residual block are a subset of transformationcoefficients of the transformation residual block.